A radio-controlled-clock (RCC) is a timekeeping device that provides the user with accurate timing information that is derived from a received signal, which is broadcast from a central location, to allow multiple users to be aligned or synchronized in time. Colloquially, these are often referred to as “atomic clocks” due to the nature of the source used to derive the timing at the broadcasting side. In the United States, the National Institute of Standards and Technology (NIST) provides such broadcast in the form of a low-frequency (60 kHz) digitally-modulated signal that is transmitted at high power from radio station WWVB in Fort Collins, Colo. The information encoded in this broadcast includes the official time of the United States. This also includes information regarding the timing of the implementation of daylight saving time (DST), which has changed in the United States over the years due to various considerations.
Similar services operating at low frequencies exist in other regions worldwide, including Europe and Japan. Many consumer-market products exist, including watches, alarm clocks and wall clocks, that are capable of receiving one or more of these broadcasts, and can display the correct time to within about one second of accuracy. While the broadcast may be active continuously, a typical radio-controlled clock may be set to receive the broadcast only once a day. Such reception, if successful (depending on the condition of the wireless link and potential interference), is typically used to reset the timekeeping device, such that if it were set incorrectly or has drifted away from the correct time, it will be set in accordance to the time communicated in the broadcast signal. Naturally, more infrequent reception of this signal, e.g., performed only once a week rather than daily may result in greater timing drifts that would be experienced in the timekeeping device This would, however, allow for energy savings, which may be crucial in a battery-operated device.
The natural drift that could be experienced in a timekeeping device, such as a wrist-watch, with respect to the accurate atomic source, which is used for the official timekeeping and broadcast, would depend on the characteristics of its frequency source, typically being based on a crystal oscillator. A common crystal that is used in such devices is one tuned to 32.768 kHz, for which a division by 215 provides a rate of 1 Hz or one pulse per second. These widely available low-cost crystals, however, may experience inaccuracies that range from a few parts per million (ppm) to ±20 ppm, depending on aging effects and the temperature range in which they operate. For example, an uncompensated timekeeping device, whose crystal resonates at 10 ppm above the nominal 32.768 kHz, will accumulate a timing drift of about 26 seconds ahead of the actual time in the course of a month, since there are about 2.6 million seconds in a month.
One purpose of the reception of the broadcast in these devices is to ensure that such drifts are compensated for before they become intolerable, although existing consumer-products do not allow for the user to specify the tolerable time-drift and it is assumed to be on the order of one second.
A second purpose for the reception of this broadcast is to ensure that the correct state of DST is assumed in the device at all times. For example, if a timekeeping device were to assume that at 2 AM in the morning of the last Sunday in October the time is to be shifted back to 1 AM, in accordance with the historical schedule for this transition, whereas the more recently instated schedule for that DST transition is 2 AM on the first Sunday of November, then the time in the device would be incorrectly set to one hour behind the actual time for a whole week. By receiving the information regarding the actual schedule for the DST transitions from this broadcast, e.g., a month ahead of time, the correctness of the time that is set in the timekeeping device can be ensured without having to receive the broadcast on the day the DST transition is to be applied.